Radio communication called “millimeter wave” can realize higher communication speed by use of high frequency electromagnetic waves. The main uses of millimeter-wave communication include short distance radio access communications, image transmission systems, simplicity radios, and automotive anti-collision radars. Also, at present, technological developments on millimeter-wave communication aimed at promoting its usage are underway, such as realization of large-capacity/long-distance transmission, downsizing of radio apparatus, and reduced cost. Here, the wavelength of millimeter waves is 10 mm to 1 mm, which corresponds to 30 GHz to 300 GHz in terms of frequency. For example, in the case of radio communication using the 60-GHz band, since channels can be allocated in GHz units, very high speed data communication can be performed.
Even compared with microwaves which are in widespread use in the wireless LAN (Local Area Network) technology or the like, millimeter waves are short in wavelength and exhibit high rectilinearity, and can transmit a very large amount of information. On the other hand, since millimeter waves are prone to severe attenuation due to reflection, the radio paths over which to perform communication are mainly direct waves, or reflected waves that are reflected once at most. Also, millimeter waves have such property that the radio signal does not reach far due to large propagation loss.
To compensate for this flight distance problem of millimeter waves, one conceivable method is to impart the antennas of a transmitter and a receiver with directivity, and direct their transmit beam and receive beam in the direction in which the communicating party is positioned to thereby extend the signal reaching distance. The beam directivity can be controlled by, for example, providing each of the transmitter and receiver with a plurality of antennas, and varying the transmit weight or receive weight for each of the antennas. Since reflected waves are rarely used and direct waves become important for millimeter waves, a beam-shaped directivity is suitable, and it is conceivable to use a sharp beam as the directivity. More preferably, radio signals are transmitted and received while directing each of the transmit beam and receive beam toward the communicating party.
For example, there has been proposed a radio transmission system in which, after the direction of the transmit antenna is determined by transmitting a signal for determining the direction of directivity of the transmit antenna by a second communication means using communication based on one of power line communication, optical communication, and acoustic communication, radio communication between the transmitter and the receiver is performed by a first communication means using radio waves of 10 GHz or more (see, for example, PTL 1).
Also, a method of extending the signal reaching distance by using the directivity of an antenna is also applied to IEEE802.15.3c, which is the standard specification of wireless PAN (mmWPAN: millimeter-wave Wireless Personal Area Network) using the millimeter-wave band.
As a technique for training of the optimal directivity of an antenna, a common method is to send a training signal from the transmitting end, and determine an optimal directivity at the receiving end in accordance with the result of its reception. For example, it is possible to vary the directivity of the antenna at the transmitting end at every transmission/reception of a single frame, and determine an optimal directivity at the receiving end in accordance with the results of frame reception.
In the case of uses in mobile environments with many moving objects, even if a directional link between communication stations is once established, there is a possibility that the directional link becomes invalid due to the subsequent movement of the communication stations or the presence of obstacles. That is, in mobile environments, it is difficult to use directional millimeter-wave communication. For this reason, one conceivable mode of operation is such that directional frames are not used when re-establishing a link, and transmission of directional frames is started after a directivity training process is conducted by using omni-directional frames. For example, in the case of adopting a collision avoidance procedure utilizing RTS/CTS handshake in millimeter-wave communication, preparation frames such as an RTS frame and a CTS frame are transmitted and received omni-directionally without using directivity, and after the direction of directivity is determined on the basis of the condition of their reception, data frames are then transmitted and received by using the directivity.
However, if a training process for an optimal directivity is executed anew every time a link is re-established, this introduces overhead until transmission of data frames is started. Also, if directivity is not used in millimeter-wave communication, this can possibly lead to a situation where due to the resulting short signal reaching distance, preparation frames such as an RTS frame and a CTS frame do not reach the communicating party (or cannot be received by the communicating party) and, as a result, transmission of data frames is never started (or directional communication cannot be started).